1. Field of the Invention
The present invention relates to a connection device for connecting between electrical devices.
2. Description of the Related Art
USB is a typical serial interface for connecting between a host computer and a peripheral device. USB is established as a general-purpose serial interface standard. In general, a host computer is referred to as a USB host, and a peripheral device is referred to as a USB device.
USB devices are classified into a bus-powered type which receives power from a host computer via the VBUS of a USB port, and a self-powered type which uses its own power supplied from a commercial power source, a battery, or the like. The bus-powered USB device is advantageous in that power from a host computer is available, but the USB standard defines the upper limit of a current value to 500 mA. On the other hand, the self-powered USB device, which uses its own power supply, has some degree of freedom in available current values.
If a hub for expanding the number of USB ports of a host computer so as to enable connection of a plurality of USB devices to the host computer is configured as a general bus-powered one, the hub sometimes limits current supplied to each of the downstream USB ports to approximately 100 mA. This is because current to be supplied from the host computer to the hub is limited to 500 mA at the maximum, and hence it is impossible to allow a maximum current of 500 mA to be supplied to each of the downstream USB devices. If the supply of the maximum current is allowed, a case will occur in which the operations of the USB devices connected to the hub cannot be guaranteed.
When it is desired to allow each USB device downstream of the hub to use current up to the maximum of 500 mA, a solution is to configure the hub as a self-powered one. A general hub, if configured as a self-powered one, can be provided with a power supply separately from the power supply of a host computer, which makes it possible to configure the hub such that current is distributed to each of USB ports up to the maximum of 500 mA.
In addition to the above-described hub configurations, there has conventionally been proposed a technique of configuring a hub for both of a self-powered mode and a bus-powered mode and switching between the two modes according to the situation. For example, a hub is provided with a detection circuit for detecting a self-powered power supply, and if there is a power supply e.g. from the commercial power source, the hub supplies power via a power path of the self-powered power supply, whereas if not, the hub supplies power via a power path of a bus-powered power supply (see e.g. Japanese Patent Laid-Open Publication No. 2002-94539).
Further, there has been proposed a technique of switching between a self-powered power supply and a bus-powered power supply according to the maximum current that is to be consumed by each of USB devices connected to a hub and the bus power supply capability of a host computer (see e.g. Japanese Patent Laid-Open Publication No. 2004-70785).
These techniques enable a hub to dynamically employ an optimal power supply configuration according to its own power supply configuration and the situation of USB devices connected thereto.
On the other hand, USB has come to be used in OA devices, such as multifunction peripherals equipped with a scan function, a print function, and so forth. Some multifunction peripherals are provided with a function of outputting data read from an original document to a USB memory and reading image data stored in the USB memory to print out the same.
Further, a multifunction peripheral has connected thereto by USB an IC card-authenticating card reader for the management of the use of Office Automation (OA) devices, a keyboard for facilitating character input which is troublesome if a touch panel is used, and so forth.
When a multifunction peripheral uses a plurality of devices, such as the card reader and the keyboard, in combination, it is desirable that the multifunction peripheral itself (i.e. the main unit thereof) is provided with a plurality of USB ports. However, functions of the multifunction peripheral, which users desire to use, differ from one user to another.
Therefore, it is desirable that each user can select a configuration of the multifunction peripheral. For this reason, a multifunction peripheral is configured, for example, such that only a single USB port is provided in the main unit thereof, and a hub circuit is provided as a separate unit from the main unit such that it can be attached to the USB port afterwards as a hub unit.
The thus configured multifunction peripheral is capable of meeting the needs of users who necessitate multiple functions, without placing extra costs on users who use only limited functions.
It is desirable for users that a hub unit is configured such that each of USB ports thereof can guarantee up to 500 mA which is the upper limit defined by the USB standard, and this is also the case with a hub unit provided for a multifunction peripheral. From this viewpoint, it is desirable that the hub unit of a multifunction peripheral is configured to be self-powered.
Further, differently from a general hub, the hub unit of a multifunction peripheral is demanded to be so configured that it is supplied with power from a host computer, i.e. the main unit of the multifunction peripheral, even when the hub unit is self-powered. This is because although the hub unit of a multifunction peripheral may be supplied with power from the commercial power source, this is not preferable due to the inconvenience of connecting an AC power supply chord to the hub unit as a separate single unit and the necessity of laying out the cord.
The hub unit of a multifunction peripheral is often disposed close to the console section of the multifunction peripheral. This arrangement takes into consideration the convenience of a user who performs attaching and removing operations when a USB memory or the like is used by being connected to the multifunction peripheral.
In the case of a large-size multifunction peripheral configured in view of the above-mentioned points, if a unit as a power supply source is disposed far from the console unit of the main unit of the multifunction peripheral, a power path from the power supply source of the main unit to the hub unit becomes long.
In such a case, the electrical resistance component of wiring inevitably increases. Therefore, as the value of current flowing through the wiring increases, the amount of voltage drop increases in proportion to the length of the power path. As a consequence, a voltage drop can occur in self-powered power supply.
Even in the hub unit used in the above-described situation, it is required to prevent voltage to be supplied from being lowered to a level below the USB standard (5V±5%) even when a rated current of the hub flows, i.e. even when a maximum voltage drop that can be expected occurs.
However, when the supply voltage of the power supply source is increased in the main unit of the multifunction peripheral or when a power supply circuit is added to the hub unit, the component costs of the main unit of the multifunction peripheral or the hub unit increase, and the degree of difficulty in device design also increases. Particularly when the supply voltage of the power supply source is increased in the main unit of the multifunction peripheral, if the voltage is also for use by another component unit of the multifunction peripheral, it is required to re-examine the degrees of influence of the increase on the units, and hence a burden on device design increases. Further, when the number of wiring members is increased or low-resistance wiring members are used so as to reduce the resistance components of cables forming a power path, material costs increase, and the price of the product becomes high.